Robust Header Compression C. Bormann
Internet-Draft Universitaet Bremen TZI
Expires: April 15, 2007 Z. Liu
Nokia Research Center
R. Price
Cogent Defence and Security
Networks
G. Camarillo
Ericsson
October 12, 2006
Applying Signaling Compression (SigComp) to the Session Initiation
Protocol (SIP)
draft-ietf-rohc-sigcomp-sip-03.txt
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Abstract
This document describes some specifics that apply when Signaling
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Compression (SigComp) is applied to the Session Initiation Protocol
(SIP), such as default minimum values of SigComp parameters,
compartment and state management, and a few issues on SigComp over
TCP. Any implementation of SigComp for use with SIP must conform to
this document, in addition to SigComp and support of the SIP and
Session Description Protocol (SDP) static dictionary.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Minimum Values of SigComp Parameters for SIP/SigComp . . . . . 3
3.1. decompression_memory_size (DMS) for SIP/SigComp . . . . . 4
3.2. state_memory_size (SMS) for SIP/SigComp . . . . . . . . . 4
3.3. cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . . 5
3.4. SigComp_version (SV) for SIP/SigComp . . . . . . . . . . . 5
3.5. locally available state (LAS) for SIP/SigComp . . . . . . 5
4. Delimiting SIP Messages and SigComp Messages on the Same
Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Continuous Mode over TCP . . . . . . . . . . . . . . . . . . . 6
6. Compartment and State Management for SIP/SigComp . . . . . . . 6
6.1. Remote Application Identification . . . . . . . . . . . . 7
6.2. Identifier Comparison Rules . . . . . . . . . . . . . . . 9
6.3. Compartment Opening and Closure . . . . . . . . . . . . . 10
6.4. Compartment Valid During a Transaction . . . . . . . . . . 11
6.5. Compartment Valid During a Registration . . . . . . . . . 11
6.6. Compartment Valid During a Dialog . . . . . . . . . . . . 12
7. Recommendations for Network Administrators . . . . . . . . . . 12
8. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 13
9. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 13
10. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Shim header for sending uncompressed messages . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
SigComp [RFC3320] is a solution for compressing messages generated by
application protocols. Although its primary driver is to compress
SIP [RFC3261] messages, the solution itself has been intentionally
designed to be application agnostic so that it can be applied to any
application protocol. (This is denoted as ANY/SigComp.)
Consequently, many application dependent specifics are left out of
the base standard. It is intended that a separate specification is
used to describe those specifics when SigComp is applied to a
particular application protocol.
This document binds SigComp and SIP (denoted as SIP/SigComp). Any
SigComp implementation that is used for the compression of SIP
messages must conform to this document, as well as to [RFC3320].
Additionally, it must support the SIP/SDP static dictionary as
specified in [RFC3485] and the mechanism for discovering SigComp
support at the SIP layer as specified in [RFC3486].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119].
3. Minimum Values of SigComp Parameters for SIP/SigComp
In order to support a wide range of capabilities among endpoints
implementing SigComp, SigComp defines a few parameters to describe
SigComp behavior (see section 3.3 of [RFC3320]). For each parameter,
[RFC3320] specifies a minimum value that any SigComp endpoint MUST
support for ANY/SigComp. Those minimum values were determined with
the consideration of all imaginable devices in which SigComp may be
implemented. Scalability was also considered as a key factor.
However, some of the minimum values specified in [RFC3320] are too
small to allow good performance for SIP message compression.
Therefore, they are increased for SIP/SigComp as specified in the
following sections. For completeness, those parameters that are the
same for SIP/SigComp as they are for ANY/SigComp are also listed.
Note: the new minimum values are specific to SIP/SigComp. They do
not apply to any other application protocols.
Note: a SigComp endpoint MAY offer additional resources if available;
these resources can be advertised to remote endpoints as described in
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section 9.4.9 of [RFC3320].
3.1. decompression_memory_size (DMS) for SIP/SigComp
Minimum value for ANY/SigComp: 2048 bytes, as specified in section
3.3.1 of [RFC3320].
Minimum value for SIP/SigComp: 8192 bytes.
Reason: a DMS of 2048 bytes is too small for SIP message compression,
as it seriously limits the compression ratio and even makes
compression impossible for certain messages. For example, the
condition set by [RFC3320] for SigComp over UDP means: C + 2*B + R +
2*S + 128 < DMS (each term is described below). On the other hand,
8KB additional memory should not cause any problem for an endpoint
that already implements SIP, SigComp, and applications that use SIP,
as DMS is memory only temporarily needed during decompression of a
SigComp message (the memory can be reclaimed when the message has
been decompressed).
C size of compressed application message, depending on R
B size of bytecode (note: two copies -- one as part of the SigComp
message and one in UDVM memory)
R size of ring buffer in UDVM memory
S any additional state uploaded other than that created from the
content of the ring buffer at the end of decompression (similar to
B, two copies of S are needed)
128 the smallest address in UDVM memory to copy bytecode to
3.2. state_memory_size (SMS) for SIP/SigComp
Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in
section 3.3.1 of [RFC3320].
Minimum value for SIP/SigComp: 2048 bytes.
Reason: a non-zero SMS allows an endpoint to upload a state in the
first SIP message sent to a remote endpoint without the uncertainty
of whether or not it can be created in the remote endpoint. A non-
zero SMS obviously requires the SIP/SigComp implementation to keep
state. Based on the observation that there is little gain from
stateless SigComp compression, the assumption is that purely
stateless SIP implementations are unlikely to provide a SigComp
function. Stateful implementations should have little problem to
keep 2K additional state for each compartment (see Section 6).
Note: SMS is a parameter that applies to each individual compartment.
An endpoint MAY offer different SMS values for different compartments
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as long as the SMS value is not less than 2048 bytes.
Compressors that make use of initial state memory MUST implement the
SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077]. (Note
that there is no such requirement on decompressors, but see also
Section 6.) For this requirement, initial state memory is defined as
the assumption of a non-zero SMS value before having received an
advertisement of non-zero SMS (e.g., via returned parameters as
specified in section 9.4.9 of [RFC3320]); ANY/SigComp as defined in
[RFC3320] does not have initial state memory.
3.3. cycles_per_bit (CPB) for SIP/SigComp
Minimum value for ANY/SigComp: 16, as specified in section 3.3.1 of
[RFC3320].
Minimum value for SIP/SigComp: 16 (same as above)
3.4. SigComp_version (SV) for SIP/SigComp
For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC3320].
For SIP/SigComp: >= 0x02 (at least SigComp + NACK)
3.5. locally available state (LAS) for SIP/SigComp
Minimum LAS for ANY/SigComp: none, see section 3.3.3 of [RFC3320].
Minimum LAS for SIP/SigComp: the SIP/SDP static dictionary as defined
in [RFC3485].
4. Delimiting SIP Messages and SigComp Messages on the Same Port
In order to limit the number of ports required by a SigComp-aware
endpoint, it is possible to allow both SigComp messages and 'vanilla'
SIP messages (i.e. uncompressed SIP messages with no SigComp header)
to arrive on the same port.
For a message-based transport such as UDP or SCTP, this can be done
per message. The receiving endpoint checks the first octet of the
UDP/SCTP payload to determine whether the message has been compressed
using SigComp. If the MSBs of the octet are "11111" then the message
is considered to be a SigComp message and is parsed as per [RFC3320].
If the MSBs of the octet take any other value, then the message is
assumed to be an uncompressed SIP message, and is passed directly to
the application with no further effect on the SigComp layer.
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For a stream-based transport such as TCP, the distinction is per
connection. The receiving endpoint checks the first octet of the TCP
data stream to determine whether the stream has been compressed using
SigComp. If the MSBs of the octet are "11111" then the stream is
considered to contain SigComp messages and is parsed as per
[RFC3320]. If the MSBs of the octet take any other value, then the
stream is assumed to contain uncompressed SIP messages, and is passed
directly to the application with no further effect on the SigComp
layer. Note that SigComp message delimiters MUST NOT be used if the
stream contains uncompressed SIP messages.
Applications MUST NOT mix SIP messages and SigComp messages on a
single TCP connection. If the TCP connection is used to carry
SigComp messages then all messages sent over the connection MUST have
a SigComp header and be delimited by the use of 0xFFFF as described
in [RFC3320].
Note: Appendix A shows how to send uncompressed messages in a SigComp
structured TCP connection using a "well-known shim header". Should
it for any reason not be desirable to set up more than one TCP
connection to a SIP implementation, but the flexibility to send both
compressed and uncompressed SIP messages be required, the compressor
can set up a SigComp structured connection and send any uncompressed
SIP messages using the well-known shim header.
5. Continuous Mode over TCP
Continuous Mode is a special feature of SigComp, which is designed to
improve the overall compression ratio for long-lived connections.
Its use requires pre-agreement between the SigComp compressor and
decompressor. Continuous mode is not used with SIP/SigComp.
Reason: continuous mode requires the transport itself to provide a
certain level of protection against denial of service attacks. TCP
alone is not considered to provide enough protection.
6. Compartment and State Management for SIP/SigComp
An application exchanging compressed traffic with a remote
application has a compartment that contains state information needed
to compress outgoing messages and to decompress incoming messages.
To increase the compression efficiency, the application must assign
distinct compartments to distinct remote applications.
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6.1. Remote Application Identification
SIP/SigComp applications identify remote applications by their SIP/
SigComp identifiers. Each SIP/SigComp application MUST have SIP/
SigComp identifier URN (Uniform Resource Name) that uniquely
identifies the application. Usage of a URN provides a persistent and
unique name for the SIP/SigComp identifier. It also provides an easy
way to guarantee uniqueness. This URN MUST be persistent across
power cycles of the device or devices hosting the SIP/SigComp
application. The SIP/ SigComp identifier MUST NOT change as the
application moves from one network to another.
A SIP/Sigcomp application SHOULD use a UUID (Universally Unique
IDentifier) URN as its SIP/SigComp identifier. The UUID URN
[RFC4122] allows for non-centralized computation of a URN based on
time, unique names (such as a MAC address), or a random number
generator. If a URN scheme other than UUID is used, the URN MUST be
selected such that the application can be certain that no other SIP/
SigComp application would choose the same URN value.
Note that the definition of SIP/SigComp identifier is similar to the
definition of instance identifier in [I-D.ietf-sip-outbound]. One
difference is that instance identifiers are only required to be
unique within their AoR (Address of Record) while SIP/SigComp
identifiers are required to be globally unique.
Nevertheless, devices may choose to generate globally unique instance
identifiers. A device with a globally unique instance identifier
SHOULD use its instance identifier as its SIP/SigComp identifier.
Server farms that share SIP/SigComp state across servers MUST use the
same SIP/SigComp identifier for all their servers.
SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI
(Uniform Resource Identifier) or Via header field parameter. The
'sigcomp-id' SIP URI parameter is a 'uri-parameter', as defined by
the SIP ABNF (Augmented Backus-Naur Form, Section 25.1 of [RFC3261]).
The following is its ABNF [RFC4234]:
sip-sigcomp-id = "sigcomp-id=" instance-val
instance-val = *uric ; defined in RFC 2396
The Via 'sigcomp-id' parameter is a 'via-extension', as defined by
the SIP ABNF (Section 25.1 of [RFC3261]). The following is its ABNF
[RFC4234]:
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via-sip-sigcomp-id = "sigcomp-id" EQUAL
LDQUOT "<" instance-val ">" RDQUOT
instance-val = *uric ; defined in RFC 2396
The following is an example of a Via header field with a 'sigcomp-id'
parameter:
Via: SIP/2.0/UDP server1.example.com:5060
;branch=z9hG4bK87a7
;comp=sigcomp
;sigcomp-id="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
Note that some characters that are allowed to appear in a Via header
field parameter, such as ':' (colon), are not allowed to appear in a
SIP URI parameter. Those characters need to be escaped when they
appear in a SIP URI parameter.
The need to escape characters in parameters could be avoided by
defining Contact, Route, Record-Route, Path, and Service-Route
header field 'sigcomp-id' parameters instead of the 'sigcomp-id'
SIP URI parameter. For example, instance identifiers typically
appear in '+sip.instance' Contact header field parameters, and not
in SIP URI parameters. We have chosen to define 'sigcomp-id' as a
SIP URI parameter to be consistent with the use of the already-in-
use 'comp=sigcomp' parameter, which is a SIP URI parameter as
well.
The following is an example of a 'sigcomp-id' SIP URI parameter:
sigcomp-id:urn%3auuid%3a0C67446E-F1A1-11D9-94D3-000A95A0E128
SIP messages are matched with remote application identifiers as
follows.
Outgoing requests: the remote application identifier is the SIP/
SigComp identifier of the URI to which the request is sent. If
the URI does not contain a SIP/SigComp identifier but is
associated with an instance identifier (e.g., the URI appears in a
Contact header field with a '+sip.instance' parameter), the
instance identifier is used as the remote application identifier.
In other cases, the remote application identifier is the host part
of the URI to which the request is sent (this is to support legacy
SIP/SigComp applications).
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Incoming responses: the remote application identifier is the same as
the one of the previously-sent request that initiated the
transaction the response belongs to.
Incoming requests: the remote application identifier is the SIP/
SigComp identifier of the top-most Via entry. If the Via header
field does not contain a SIP/SigComp identifier, the remote
application identifier is the sent-by parameter of the top-most
Via entry (this is to support legacy SIP/SigComp applications).
Outgoing responses: the remote application identifier is the same as
the previously-received request that initiated the transaction the
response belongs to.
A SIP/SigComp application placing its URI with the 'comp=sigcomp'
parameter in a header field MUST add a 'sigcomp-id' parameter with
its SIP/SigComp identifier to that URI unless the URI is associated
with an instance identifier (e.g., the URI appears in a Contact
header field with a '+sip.instance' parameter). If the URI is
associated with an instance identifier, the SIP/SigComp application
SHOULD NOT add a 'sigcomp-id' parameter to the URI.
A SIP/SigComp application generating its own Via entry containing the
'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its
SIP/SigComp identifier to that Via entry.
A given remote application identifier is mapped to a particular
SigComp compartment ID following the rules given in Section 6.3,
Section 6.4, Section 6.5, Section 6.6.
6.2. Identifier Comparison Rules
Equality comparisons between SIP/SigComp identifiers are performed
using the rules for URN equality that are specific to the scheme in
the URN. If the element performing the comparisons does not
understand the URN scheme, it performs the comparisons using the
lexical equality rules defined in RFC 2141 [RFC2141]. Lexical
equality may result in two URNs being considered unequal when they
are actually equal. In this specific usage of URNs, the only element
which provides the URN is the SIP/SigComp application identified by
that URN. As a result, the SIP/SigComp application SHOULD provide
lexically equivalent URNs in each registration it generates. This is
likely to be normal behavior in any case; applications are not likely
to modify the value of their SIP/SigComp identifiers so that they
remain functionally equivalent yet lexigraphically different from
previous identifiers.
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Comparisons between SIP/SigComp identifiers and instance identifiers
are performed following the same rules.
6.3. Compartment Opening and Closure
SIP applications need to know when to open a new compartment and when
to close it. The lifetime of a compartment depends on how the SIP
application obtained the remote application identifier (e.g., in a
Record-Route header field of an incoming SIP message). There are
compartments that are valid for the duration of a registration, of a
dialog, and of a single transaction. The following sections specify
how a SIP application decides the lifetime of a particular
compartment.
If following the rules in the following sections, a SIP application
is supposed to open a compartment for a remote application identifier
for which it already has a compartment, the SIP application MUST use
the already existing compartment. That is, the SIP application MUST
NOT open a new compartment. Additionally, the SIP application MUST
adjust the closure time for the compartment so that it is only closed
when the SIP application does not need it any longer.
For example, a SIP application may open a compartment valid for the
duration of a registration for a particular remote application
identifier. At a later point, the application is supposed to open a
new compartment for the duration of a particular dialog for the same
remote application identifier. Following the previous rule, the SIP
application does not open a new compartment but use the already
existing one for that remote application identifier. However, the
SIP application must not close that compartment until both, the
registration and the dialog are over. So, if the registration
finishes before the dialog, the compartment will not be closed
(because the dialog is still active) even though the compartment was
originally opened for the registration.
Usually, any states created during the lifetime of a compartment will
be "logically" deleted when the compartment is closed. As described
in section 6.2 of [RFC3320], a logical deletion can become a physical
deletion only when no compartment continues to exist that created the
(same) state.
A SigComp endpoint may offer to keep a state created upon request
from a SigComp peer endpoint beyond the default lifetime of a
compartment. This may be used to improve compression efficiency of
subsequent SIP messages generated by the same remote application at
the SigComp peer endpoint. To indicate that such state will continue
to be available, the SigComp endpoint can inform its peer SigComp
endpoint by announcing the (partial) state ID in the returned SigComp
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parameters at the end of the registration, dialog, or transaction
that was supposed to limit the lifetime of the SigComp state. That
signals the state will be maintained. As there is no way to signal
any limit to the lifetime of this state, both decompressors that
intend to offer state with possibly limited lifetimes as well as
compressors that make use of such state use the SigComp Negative
Acknowledgement (NACK) Mechanism [RFC4077] to recover from
synchronization errors.
As an operational concern, bugs in the compartment management
implementation are likely to lead to sporadic, hard to diagnose
failures. Decompressors may therefore want to cache old state and,
if still available, allow access while logging diagnostic
information. Both compressors and decompressors use the SigComp
Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from
situations where such old state may no longer be available.
6.4. Compartment Valid During a Transaction
A SIP application that needs to send a compressed SIP request SHOULD
open a compartment for the request's remote application identifier.
This compartment will be used to receive compressed responses for the
request. The application SHOULD NOT close the compartment until the
transaction is over.
A SIP application that receives a compressed SIP request SHOULD open
a compartment for the request's remote application identifier. This
compartment will be used to send compressed responses for the
request. The application SHOULD NOT close the compartment until the
transaction is over.
The previous rules ensure that SIP applications always have a
compartment to send and receive responses.
6.5. Compartment Valid During a Registration
A REGISTER transaction can cause an application to open a new
compartment to be valid for the duration of the registration
established by the REGISTER transaction.
A 200 (OK) response for a register may contain a Path [RFC3327] and a
Service-Route [RFC3308] header field. These header fields indicate
the route future incoming and outgoing requests will follow.
A SIP application generating a 200 (OK) response for a REGISTER or
receiving such a response proceeds as follows. If the application
inserted itself in the Contact (i.e., because it is the user agent)
or in the Path header field of the REGISTER, or it appears in the
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Service-Route header field, the application constructs the route
future incoming requests will follow (using the Contact and the Path
header fields) and the route future outgoing requests will follow
(using the Contact and the Service-Route header fields). The
application checks whether the URIs of its adjacent applications in
both routes have the 'comp=sigcomp' parameter. The application
SHOULD open a new compartment for the remote application identifier
of the URIs with that parameter. The application SHOULD NOT close
the compartments until the registration is over.
Note that the route for incoming requests is typically the same
(although traversed in the opposite direction) as the route for
outgoing requests.
6.6. Compartment Valid During a Dialog
A transaction that establishes a dialog can cause an application to
open a new compartment to be valid for the duration of the dialog
established by the transaction.
A SIP message that establishes a dialog (e.g., a 2xx response for an
INVITE) may contain a Record-Route header field. This header field
indicates the route future requests within the dialog will follow.
On generating or receiving a SIP message that establishes a dialog, a
SIP application that inserted itself in the Contact (i.e., because it
is the user agent) or in the Record-Route header field of the
request, constructs (using the Contact, and the Record-Route header
fields) the route requests within the dialog will follow. The
application checks whether the URIs of its adjacent applications in
that route have the 'comp=sigcomp' parameter. The application SHOULD
open a new compartment for the remote application identifier of the
URIs with that parameter. The application SHOULD NOT close the
compartments until the dialog is over.
7. Recommendations for Network Administrators
Network administrators can configure their networks so that the
compression efficiency achieved is increased. The following
recommendations help network administrators perform their task.
For a given user agent, the route sets for incoming requests (created
by a Path header field) and for outgoing requests (created by a
Service-Route header field) are typically the same. However,
registrars can, if they wish, insert proxies in the latter route that
do not appear in the former route and vice versa. It is RECOMMENDED
that registrars are configured so that proxies performing SigComp
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compression appear in both routes.
The routes described previously apply to requests sent outside a
dialog. Requests inside a dialog follow a route constructed using
Record-Route header fields. It is RECOMMENDED that the proxies
performing SigComp that are in the route for requests outside a
dialog are configured to place themselves (by inserting themselves in
the Record-Route header fields) in the routes used for requests
inside dialogs.
8. Private Agreements
SIP/SigComp implementations that are subject to private agreements
MAY deviate from this specification, if the private agreements
unambiguously specify so. Plausible candidates for such deviations
include:
o Minimum values (Section 3).
o Compartment definition (Section 6).
o Use of continuous mode (Section 5).
9. Backwards Compatibility
SigComp has a number of parameters that can be configured per
endpoint. This document specifies a profile for SigComp when used
for SIP compression that further constrains the range that some of
these parameters may take. Examples of this are Decompressor Memory
Size, State Memory Size, and SigComp Version (support for NACK).
Additionally, this document specifies how SIP/SigComp applications
should perform compartment mapping.
When this document was written, there already were a few existing
SIP/SigComp deployments. The rules in this document have been
designed to maximize interoperability with those legacy SIP/SigComp
implementations. Nevertheless, implementers should be aware that
legacy SIP/SigComp implementations may not conform to this
specification. Examples of problems with legacy applications would
be smaller DMS than mandated in this document, lack of NACK support,
or a different comparment mapping.
10. Example
Figure 1 shows an example message flow where the user agent and the
outbound proxy exchange compressed SIP traffic. Compressed messages
are marked with a (c).
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User Agent Outbound Proxy Registrar
|(1) REGISTER (c) | |
|---------------->| |
| |(2) REGISTER |
| |---------------->|
| |(3) 200 OK |
| |<----------------|
|(4) 200 OK (c) | |
|<----------------| |
|(5) INVITE (c) | |
|---------------->| |
| |(6) INVITE |
| |------------------------------>
| |(7) 200 OK |
| |<------------------------------
|(8) 200 OK (c) | |
|<----------------| |
|(9) ACK (c) | |
|---------------->| |
| |(10) ACK |
| |------------------------------>
|(11) BYE (c) | |
|---------------->| |
| |(12) BYE |
| |------------------------------>
| |(13) 200 OK |
| |<------------------------------
|(14) 200 OK (c) | |
|<----------------| |
Figure 1: Example message flow
The user agent in Figure 1 is initialy configured (e.g., using the
SIP configuration framework [I-D.ietf-sipping-config-framework]) with
the URI of its outbound proxy. That URI contains the outbound's
proxy SIP/SigComp identifier, referred to as 'Outbound-id', in a
'sigcomp-id' parameter.
When the user agent sends an initial REGISTER request (1) to the
outbound proxy's URI, the user agent opens a new compartment for
'Outbound-id'. This compartment will, in principle, be valid for the
duration of the REGISTER transaction, as discussed in Section 6.4.
On receiving this REGISTER request (1), the outbound proxy opens a
new compartment for the SIP/SigComp identifier that appears in the
'sigcomp-id' parameter of the top-most Via entry. This identifier,
which is the user agent's SIP/SigComp identifier, is referred to as
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'UA-id'. The compartment opened by the outbound proxy will, in
principle, be valid for the duration of the REGISTER transaction, as
discussed in Section 6.4. The outbound proxy adds Path header field
with its own URI to the REGISTER request and relays it to the
registrar (2).
When the outbound proxy receives a 200 (OK) response (3) for the
REGISTER request, it constructs the route future incoming requests
will follow (using the Contact and the Path header fields) and the
route future outgoing requests will follow (using the Contact and the
Service-Route header fields). Both the Path and the Service-Route
header fields contain the outbound proxy's URI. The Contact header
field contains the user agent's URI, which carries the user agent's
SIP/SigComp identifier 'UA-id'.
Consequently, the outbound proxy is supposed to open a new
compartment for 'UA-id' for the duration of the registration, as
discussed in Section 6.4. However, since the outbound proxy has
already a compartment for 'UA-id', it reuses that comparment, as
discussed in Section 6.3.
On receiving the 200 (OK) response (4), the user agent constructs the
route future incoming requests will follow (using the Path header
field) and the route future outgoing requests will follow (using the
Service-Route header field). The user agent is supposed to open a
new compartment for 'Outbound-id' for the duration of the
registration, as discussed in Section 6.4. However, since the user
agent has already a compartment for 'Outbound-id', it reuses that
comparment, as discussed in Section 6.3.
At a later point, the user agent needs to send an INVITE request (5).
The user agent is supposed to open a new compartment for
'Outbound-id' for the duration of the INVITE transaction, as
discussed in Section 6.4. However, since the user agent has already
a compartment for 'Outbound-id', it reuses that comparment, as
discussed in Section 6.3.
On receiving the INVITE request (5), the outbound proxy is supposed
to open a new compartment for 'UA-id' for the duration of the INVITE
transaction, as discussed in Section 6.4. However, since the
outbound proxy has already a compartment for 'UA-id', it reuses that
comparment, as discussed in Section 6.3. The outbound proxy Record-
Routes and relays the INVITE request (6) forward.
When the outbound proxy receives a dialog-establishing 200 (OK)
response (7) for the INVITE request, it constructs the route future
requests within the dialog will follow (using the Contact and Route
header fields). The outbound proxy is supposed to open a new
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compartment for 'UA-id' for the duration of the dialog, as discussed
in Section 6.6. However, since the outbound proxy has already a
compartment for 'UA-id', it reuses that comparment, as discussed in
Section 6.3.
On receiving the 200 (OK) response (8), the user agent constructs the
route future requests within the dialog will follow (using the Route
header field). The user agent is supposed to open a new compartment
for 'Outbound-id' for the duration of the dialog, as discussed in
Section 6.4. However, since the user agent has already a compartment
for 'Outbound-id', it reuses that comparment, as discussed in
Section 6.3.
When the dialog is terminated by a BYE transaction (11), the user
agent is supposed to close the compartment for 'Outbound-id' and the
outbound proxy is supposed to close the compartment for 'UA-id', as
discussed in Section 6.6. However, both compartments should be kept
open until the current registration expires. Therefore, none of them
close their compartments yet.
11. Security Considerations
The same security considerations as described in [RFC3320] apply to
this document. Note that keeping SigComp states longer than the
duration of a SIP dialog should not pose new security risks for two
reasons: a) the state has been allowed to be created in the first
place; and b) this is on voluntary basis and a SigComp endpoint can
choose not to offer it.
12. IANA Considerations
The IANA is requested to register the 'sigcomp-id' Via header field
parameter, which is defined in Section 6.1, under the Header Field
Parameters and Parameter Values subregistry within the SIP Parameters
registry:
Predefined
Header Field Parameter Name Values Reference
---------------------------- --------------- --------- ---------
Via sigcomp-id No [RFCxxxx]
The IANA is requested to register the 'sigcomp-id' SIP URI parameter,
which is defined in Section 6.1, under the SIP/SIPS URI Parameters
subregistry within the SIP Parameters registry:
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Parameter Name Predefined Values Reference
-------------- ----------------- ---------
sigcomp-id No [RFCxxxx]
Note to the RFC Editor: please, substitute RFCxxxx with the RFC
number this document will get.
13. Acknowledgements
Abigail Surtees provided the code and text for Appendix A.
The authors would like to thank the following people for their
comments and suggestions: Abigail Surtees, Jan Christoffersson, Joerg
Ott, Mark West, Pekka Pessi, Robert Sugar, Adam Roach, Jonathan
Rosenberg, and Robert Sparks.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer
Two Tunneling Protocol (L2TP) Differentiated Services
Extension", RFC 3308, November 2002.
[RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
Liu, Z., and J. Rosenberg, "Signaling Compression
(SigComp)", RFC 3320, January 2003.
[RFC3327] Willis, D. and B. Hoeneisen, "Session Initiation Protocol
(SIP) Extension Header Field for Registering Non-Adjacent
Contacts", RFC 3327, December 2002.
[RFC3485] Garcia-Martin, M., Bormann, C., Ott, J., Price, R., and A.
Roach, "The Session Initiation Protocol (SIP) and Session
Description Protocol (SDP) Static Dictionary for Signaling
Compression (SigComp)", RFC 3485, February 2003.
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[RFC3486] Camarillo, G., "Compressing the Session Initiation
Protocol (SIP)", RFC 3486, February 2003.
[RFC4077] Roach, A., "A Negative Acknowledgement Mechanism for
Signaling Compression", RFC 4077, May 2005.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006.
14.2. Informative References
[I-D.ietf-sipping-config-framework]
Petrie, D., "A Framework for Session Initiation Protocol
User Agent Profile Delivery",
draft-ietf-sipping-config-framework-08 (work in progress),
March 2006.
Appendix A. Shim header for sending uncompressed messages
This appendix presents bytecode that simply instructs the
decompressor to output the entire message (effectively sending it
uncompressed but within a SigComp message).
The mnemonic code is:
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at (0)
:udvm_memory_size pad (2)
:cycles_per_bit pad (2)
:sigcomp_version pad (2)
:partial_state_id_length pad (2)
:state_length pad (2)
:reserved pad (2)
at (64)
:byte_copy_left pad (2)
:byte_copy_right pad (2)
:input_bit_order pad (2)
:stack_location pad (2)
; Simple loop
; Read a byte
; Output a byte
; Until there are no more bytes!
at (128)
:start
INPUT-BYTES (1, byte_copy_left, end)
OUTPUT (byte_copy_left, 1)
JUMP (start)
:end
END-MESSAGE (0,0,0,0,0,0,0)
which translates to give the following initial 13 bytes of the
SigComp message (in hexadecimal):
f8 00 a1 1c 01 86 09 22 86 01 16 f9 23
As an implementation optimization, a SigComp implementation MAY
compare the initial 13 bytes of each incoming message with the 13
bytes given (the "well-known shim header"), and, in case of a match,
simply copy the SigComp message data that follow the shim header
without even setting up a UDVM. (Note that, before a SigComp message
is formed from the incoming TCP data, the record marking protocol
defined in section 4.2.2 of [RFC3320] has to be performed.)
To obtain the maximum benefit from this optimization, compressors
SHOULD employ exactly the well-known shim header given (and none of
the other conceivable byte code sequences for just copying input to
output) to send uncompressed data in a SigComp channel.
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28334
Germany
Phone: +49 421 218 7024
Fax: +49 421 218 7000
Email: cabo@tzi.org
Zhigang Liu
Nokia Research Center
6000 Connection Drive
Irving, TX 75039
USA
Phone: +1 972 894-5935
Email: zhigang.c.liu@nokia.com
Richard Price
Cogent Defence and Security Networks
Queensway Meadows Industrial Estate
Meadows Road
Newport, Gwent NP19 4SS
Phone: +44 (0)1794 833681
Email: richard.price@cogent-dsn.com
URI: http://www.cogent-dsn.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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